Vehicle operators usually prefer to drive a vehicle having a soft, smooth ride with predictable handling characteristics. These features are particularly desirable in long-haul trucks where an operator may drive a heavily loaded truck continuously for many hours.
One type of variable-stiffness suspension system frequently used with trucks incorporates leaf springs, shock absorbers, and air springs. Typical systems of this type are taught in U.S. Pat. No. 3,802,718, to Schaeff, and U.S. Pat. No. 4,946,190, to Buttner. Generally, in these systems the air springs and shock absorbers absorb the suspension forces from vertical motions while the leaf springs allow the wheels to negotiate individual bumps affecting one wheel.
While the leaf spring, shock absorber, and air spring suspension systems are generally successful, they tend to be heavy. Not only do heavy suspension systems reduce fuel economy, but since the legally permissible weight of a vehicle on most highways is limited the weight of the suspension system directly reduces the weight of material the vehicle can carry. Lighter suspensions tend to be less expensive to construct, as well. Therefore, a lighter-weight suspension system is beneficial.
A major contributor to the weight of a typical suspension system is the leaf springs. Leaf springs are usually composed of stacked sections of long, flat pieces of spring steel that are much wider than they are tall. The bending strength of a leaf spring, which is nearly rectangular in cross section, is measured by a quantity called the section modulus. The leaf spring rectangular cross section modulus is given by the formula: EQU Z=A(d)/6
wherein Z is the section modulus (in cubic inches), A is the cross-sectional area of the leaf spring, and d is the leaf spring vertical depth. For a given area, A, the leaf spring is therefore stronger when oriented so that the depth, d, is larger than the width. This is similar to using a deep vertical but horizontally narrow beam as a wood joist or rafter in home construction. Vertically oriented elongated elements are taught in U.S. Pat. Nos. 4,310,171 and 4,541,653. However, those vertically oriented, elongated spring members are rigid rather than springy and are made of common beam material to provide the strength, thereby adding weight. The rigidity of such a beam does not allow the suspension to accommodate individual bumps.
Leaf springs are usually formed with a wrapped "eye" at each end to assist attachment of the leaf spring to the vehicle. Leaf springs with wrapped eyes tend to be weak at the wrap and subject to fatigue-induced failures. Therefore, wrapped-eye leaf springs usually incorporate a backup leaf loosely wrapped around the main leaf in case the main leaf spring fails. While this construction, termed a "military wrap," provides safety, it increases the overall suspension system weight. Additionally, only one eye at a time can support fore and aft load, so the combined strength of two eyes (the wrapped eye and the military wrap) is not available.
One way of reducing the weight of the suspension system is to make the suspension of compact structural parts. Compactness was limited in most prior art suspension systems because the leaf springs are usually dimensioned to have the axle attach at their midpoints, as illustrated by U.S. Pat. No. 3,309,107, or they include additional materials to achieve desired geometries, as in U.S. Pat. No. 4,946,190. Besides a weight reduction advantage, compact suspension parts are able to better utilize the suspension components. This follows because as the available system components are brought closer together, the moment arms of the components, and hence weight, are reduced.
Therefore, there has existed a need for a lightweight, compact suspension system for a wheeled vehicle.